The present invention relates generally to devices for performing downhole operations in subterranean wells. More specifically, the invention relates to injectors for injecting coil tubing strings into subterranean wells and extracting the coil tubing strings from the subterranean wells to perform well-servicing operations.
Continuous reeled tubing, generally known in the energy industry as coil tubing string, has been used for many years. It is much faster to run into and out of a well casing than conventional jointed, tubing.
Typically, the coil tubing string is inserted into the wellhead through a lubricator assembly or stuffing box because there is a pressure differential between an annulus of the well and atmosphere, which may have been naturally or artificially created. The pressure differential serves to produce oil or gas, or mixture thereof from the pressurized well. A coil tubing string is run in and out of a well bore using a coil tubing string injector, which literally forces the coil tubing string into the well through the lubricator assembly or stuffing box against the well pressure until the weight of the coil tubing string exceeds the force of the pressure acting against a cross-sectional area of the coil tubing string. However, once the weight of the coil tubing string overbears the well pressure, it must be supported by the injector. The injection process is reversed as the coil tubing string is removed from the well.
The coil tubing string is relatively flexible and can therefore be wound onto and pulled off of a spool, or reel, by the injector, which often acts in concert with a windlass at a power supply that drives the spool, or reel. Conventionally, a coil tubing injector assembly utilizes a pair of opposed endless drive chains which are arranged in a common plane. These opposed endless drive chains are often referred to as gripper chains and carry a series of gripping blocks which are pressed against opposite sides of the coil tubing string and thereby grip the coil tubing string. Each chain is stretched between a drive sprocket and an idle sprocket. At least one of the two drive sprockets is driven by a motor to turn one of the endless chains, to supply injection or pulling force. The other drive sprocket may also be driven, typically by a second motor, to drive the second chain in order to provide extra power. Such coil tubing string injectors with various improvements are disclosed, for example, in U.S. Pat. No. 4,655,291, entitled INJECTOR FOR COUPLED PIPE, which issued to Cox on Apr. 7, 1987; U.S. Pat. No. 5,553,668, entitled TWIN CARRIAGE TUBING INJECTOR APPARATUS, which issued to Council et al. on Sep. 10, 1996; and U.S. Pat. No. 6,059,029, entitled COILED TUBING INJECTOR, which issued to Goode on May 9, 2000.
Another type of coil tubing string injector is disclosed in U.S. Pat. No. 5,566,764, entitled IMPROVED COIL TUBING INJECTOR UNIT which issued to Elliston on Oct. 22, 1996. Elliston describes a coil tubing string injector unit including a main injector frame having a longitudinal opening that defines a vertical run for the injector unit, which can be aligned with the well bore vertical axis. Elliston""s injector unit has only one gripper chain drive system that carries plier-like halves that are pivotable between an open position and a closed, gripping position as the gripper chain enters the vertical run, so that the plier halves grip a selected length of a coil tubing string fed into the main injector frame along the central vertical axis of the injector unit to inject the coil tubing string into the well bore.
The prior art known to the Applicant fails to disclose a coil tubing injector assembly that is capable of injecting dual string coil tubing into a well bore simultaneously, even though the use of tubing strings is known in the energy industry. For example, U.S. Pat. No. 4,474,236, entitled METHOD AND APPARATUS FOR REMOTE INSTALLATION OF DUAL TUBING STRINGS IN A SUBSEA
WELL, which issued to Kellett on Oct. 2, 1984, discloses a method and apparatus for completing a well having production and service strings of different sizes. The method includes steps of running the production string on a main tubing string hanger and maintaining control with a variable bore blowout preventer, and then running the service string into the main tubing string hanger while maintaining control using a dual bore blowout preventer. Use of this method and apparatus is, however, time-consuming and therefore expensive.
Therefore, there exists a need for an apparatus which is adapted to simultaneously inject dual string coil tubing into, or extract dual string coil tubing from, a well bore.
It is therefore an object of this invention to provide a coil tubing injector assembly adapted to simultaneously inject dual string coil tubing into a subterranean well, or extract the dual string coil tubing from the well.
It is another object of the invention to provide a method of running coil tubing strings into a subterranean well to permit a downhole operation to be performed using a coil tubing string injection apparatus adapted to inject dual string coil tubing, so that the dual string coil tubing may be injected synchronously or asynchronously.
In general, the present invention provides a coil tubing injector assembly that comprises a frame structure; and a gripper chain drive system mounted to the frame structure and adapted to engage first and second coil tubing strings, to inject both the first and second coil tubing strings into, and withdraw both the first and second coil tubing strings from, a subterranean well. The gripper chain drive system preferably comprises a pair of gripper chains disposed in a common plane and spaced apart from each other so that a length of the first and second coil tubing strings are temporarily engaged between, and are moved by the pair of gripper chains.
The coil tubing injector assembly in accordance with one embodiment of the invention, includes a frame structure and a pair of substantially identical gripper chain drive systems mounted to the frame structure, disposed in a common plane and spaced apart from each other to inject both a first and a second coil tubing string into, and withdraw both the first and second coil tubing strings from, a subterranean well. Each of the gripper chain drive systems includes a drive shaft and an idle shaft respectively rotatably mounted to the frame structure. The gripper chain engages a drive sprocket and an idle sprocket mounted to the respective drive and idle shafts. The gripper chain includes coil tubing string gripping blocks adapted to grip both of the coil tubing strings, and each coil tubing string gripping block has a first side for engaging the first coil tubing string and a second side for engaging the second coil tubing string. Each side has a predetermined thickness so that a secure engagement with the coil tubing strings is ensured, even if the coil tubing strings have different diameters.
A pair of pressure beams are mounted to the frame structure for supporting the respective gripper chains. The pressure beam preferably includes a roller chain system for reducing friction between the beam and the gripper chain. The respective pressure beams are movable to grip or release the first and second coil tubing strings, as required.
In accordance with another embodiment of the invention, each of the pair of gripper chain drive systems includes a first and second gripper chain drive sub-system supported by the frame structure in a parallel relationship. Each of the sub-systems includes a drive shaft, an idle shaft and a gripper chain engaged with a drive sprocket and an idle sprocket mounted to the respective drive and idle shafts. The gripper chain carries coil tubing string gripping blocks for engaging one of the coil tubing strings so that the first and second coil tubing strings are respectively engaged between, and are moved by the respective first gripper chain drive sub-systems and second gripper chain drive sub-systems.
Each of the sub-systems is equipped with a pressure beam for supporting the gripper chain when the gripper chain engages the coil tubing string. The pressure beam preferably includes a roller chain system for reducing friction between the beam and the gripper chain. The pressure beams are movable with respect to each other to support the respective gripper chains when they engage the first and second coil tubing strings.
The drive shafts of the sub-systems of each gripper chain drive system may be aligned with each other to form an integral drive shaft. If so, the sprockets mounted on the integral drive shaft have the same diameter, so that the first and second coil tubing strings are injected or withdrawn at the same speed. The idle shafts of the sub-systems of each gripper chain drive system may also be aligned axially with each other to form an integral idle shaft. The idle sprockets mounted on the integral idle shaft also have the same diameter.
In accordance with a further embodiment of the invention, the drive shafts of the pair of first gripper chain drive sub-systems are rotated synchronously in opposite directions, but independently of the drive shafts of the pair of second gripper chain drive sub-systems so that the first and second coil tubing strings may be injected independently of one another, or at different rates.
In accordance with another aspect, the invention provides a method of running coil tubing strings into a subterranean well to permit a downhole operation to be performed. The method comprises a step of injecting first and second coil tubing strings through a wellhead into the well using a coil tubing string injection apparatus adapted to inject- the first and second coil tubing strings into the well simultaneously. The first and second coil tubing strings may be injected either synchronously or asynchronously to satisfy different requirements in various applications. The coil tubing injector assembly and the method of running coil tubing strings using the coil tubing injector assembly in accordance with the invention is adapted for use in a wide variety of applications. For example, the invention enables a well stimulation process to be conducted using two coil tubing strings simultaneously. One coil tubing string is used to stimulate a production zone above a packer or a plug, while the other coil tubing string runs through the packer or plug and is used to stimulate a production zone below the packer or plug. The dual string coil tubing can also be used to stimulate separate production zones by pumping down one coil tubing string first, and then pumping down the second coil tubing string after stimulating the first zone, without repositioning the respective coil tubing strings, the packer or plug.
The invention also enables one coil tubing string to be used for stimulation, while the second coil tubing string is used to record actual downhole pressure and temperature. The invention also enables one coil tubing string to be used for well stimulation, while the other coil tubing string is used to spot fluids such as prefrac acids, etc., if required. If the first coil tubing string is used for stimulation, the second coil tubing string may be kept in reserve for cleanout, in the event of a screenout. The invention also permits the first coil tubing string to be used to stimulate the well, while the second coil tubing string is used to house electrical conductors for detonating perforating charges in a perforating/stimulation fluid injector tool.
The injector assembly in accordance with the invention can also be used to inject any flexible, seamless member into a well, such as a wireline, for example.
The features and advantages of the present invention will be better understood with reference to preferred embodiments as described below.